Embodiments of the present invention relate to coatings intended to inhibit the formation and coating systems for preventing or reducing the deposition of carbonaceous deposits on surfaces that are at elevated temperatures when contacted by hydrocarbon fluids, including rough and complex fluid passage surfaces contacted by fuels and oils.
Aircraft gas turbine engines function by receiving air through an intake, compressing the air, mixing fuel into the compressed air, combusting the fuel in the fuel/air mixture, and using the resulting hot combustion gases to propel an aircraft. Staged combustion systems have been developed for use in aircraft gas turbine engines to limit the production of undesirable combustion product components such as oxides of nitrogen (NOx), unburned hydrocarbons (HC), and carbon monoxide (CO), particularly in the vicinity of airports, where they contribute to urban photochemical smog problems. Also, gas turbine engines are designed to achieve better fuel efficiency and lower operational costs, while simultaneously maintaining or even increasing engine output. Consequently, important design criteria for aircraft gas turbine engine combustion systems include provisions for high combustion temperatures to provide high thermal efficiency under a variety of engine operating conditions and to minimize undesirable combustion conditions that can contribute to the emission of particulates, undesirable gases, and combustion products that can be precursors to the formation of photochemical smog.
The injection of fuel into the compressed air to form a fuel/air mixture in the combustion chamber is an important aspect of engine operation because the composition of the fuel/air mixture and the method of injection can have large impacts on overall engine performance. Fuel injector designs generally entail some type of fuel nozzle for injecting fuel into the combustion chamber. Fuel nozzle designs may include main and pilot nozzles, and they may include axially, radially, and circumferentially extending fuel passages that supply fuel to the main and pilot nozzles. Portions of these fuel passages may be very small and complicated with geometries such as sharp bends or spirals.
The construction and fabrication of a fuel nozzle can significantly impact the method of fuel injection. As such, improved fuel nozzles and methods for their fabrication are constantly being sought. One such method is additive manufacturing (AM). As used herein, AM refers to processes that entail fusing powders to form a solid three-dimensional net or near-net-shape (NNS) object by sequentially forming the object one layer at a time. AM processes may include, but are not limited to, three-dimensional printing (3DP) processes, laser-net-shape manufacturing (LNSM), direct metal laser melting (DMLM), and electron beam sintering. Some AM processes use energy beams, for example, electron beams or electromagnetic radiation such as laser beams, to sinter or melt a powder material. An object is built up layer by layer in a linear build direction, with each layer being substantially perpendicular to the build direction. AM processes can integrate computer-aided design (CAD) models to produce objects having complex geometries.
AM processes may be beneficial for the production of fuel nozzles, as they allow novel and complex nozzle designs to be produced and tested relatively quickly. However, components produced by AM processes tend to have rough surfaces, which in the case of fuel nozzles, includes internal passages through which fuel will flow. In some instances, the range of interior surface roughnesses of fuel passages produced by AM processes can be up to about 1200 micro-inches (about 30 micrometers) Ra or greater, for example, about 300 to about 1200 micro-inches (about 8 to 30 micrometers) Ra (the roughness parameter defined by the arithmetic average of the absolute values of the vertical deviations on the surface). Consequently, complex designs and rough interior surfaces of fuel nozzles produced in this manner present their own challenges.
Coke deposition is a common issue in aircraft fuel and lubrication systems exposed to high temperatures. Coke deposition can be caused by the catalytic-thermal degradation of hydrocarbon fluids, resulting in carbon becoming attached and building up as deposits on surfaces contacted by a fuel or oil. Carbon deposits may develop if the fluid circuit is operated at reduced flow rates or closed without the remaining stagnant fuel being purged. As the deposits collect, they can become sufficiently large to reduce or even obstruct fluid flow. In the case of a fuel circuit, such carbon deposition can lead to degraded engine performance, reduced heat transfer efficiencies, increased pressure drops, and increased rates of material corrosion and erosion, all of which can necessitate the use of expensive de-coking procedures.
Suitable countermeasures to coke build-up may include the application of a coating, sometimes referred to as a coke barrier coating (CBC) or an anti-coking coating system, to the interior surfaces of a component such as a fuel nozzle or other surface that will be at elevated temperatures when contacted by a hydrocarbon fluid. Examples of anti-coking coating systems include an inner layer, which may be a ceramic material, applied to the surface of a fluid passage, over which an outer layer, which may be platinum, is deposited that will be contacted by the fluid. The inner layer may serve as a diffusion barrier layer that separates the outer layer from the surface on which the coating system is deposited. The outer layer hinders carbon deposits from sticking to the surfaces of the fluid passage, and in some forms may serve as a catalyst to form nonadherent particles, thereby reducing coking and deposit buildup. With the coating system in place, small flakes of coke quickly spall from the passage walls with little risk of blocking small orifices or metering passages that may exist downstream. In an embodiment, the coating system is continuous and completely covers all surfaces of a component that would otherwise contact the hydrocarbon fluid. Such coating systems may further contain additional layers as long as the hydrocarbon fluid will contact the outer layer, which, in certain embodiments, may comprise or consist of platinum at the outermost surface of the coating system.
In an embodiment, to minimize the temperature of the hydrocarbon fluid and, therefore, the tendency for the fluid to form carbonaceous deposits, the outermost layer exhibits low emissivity. Such low emissivity minimizes radiation heat transfer to the fluid. For this purpose, in an embodiment, a surface roughness for the outermost layer may be about 40 micro-inches (about 1.0 micrometer) Ra or less. The inner and outermost layers of anti-coking coating systems may be applied using chemical vapor deposition (CVD) techniques, in which vapors containing one or more suitable chemical precursors may be deposited on the intended surface, where the precursors may be reacted or decomposed to form one of the desired layer materials. Because CVD processes are capable of depositing conformal layers so that the surface finish of the coating system nearly replicates that of the underlying surface, to attain a surface finish typically desired for the outermost layer of an anti-coking coating system, i.e., about 1.0 micrometer Ra or less, conventional wisdom would suggest that the surface to be coated may need to undergo a treatment to improve its surface finish, followed by deposition of the coating system whose final surface finish may be and often is only slightly better than that of the underlying surface.
While anti-coking coating systems deposited by CVD have proven effective for certain engine components, including lubricant and scavenge lines, such components have predominantly straight or only slightly curved passages and smooth interior surfaces. There exists a need for similar anti-coking systems that may be applied to fluid passages of components having complex shapes and rough interior surfaces, nonlimiting examples of which include components produced by AM processes.